The present invention relates to an image signal processing apparatus for converting a continuous tone image into a halftone dot image, and more particularly to an image signal processing apparatus for comparing an image signal obtained from a continuous tone image with a threshold signal, and generating an image signal indicative of a half-tone dot image based on a probabilistically established signal if the difference between the image signal and the threshold signal falls within a predetermined range.
Image scanning reading and reproducing systems are widely used in the printing and platemaking industries for electrically processing image information of originals or subjects to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reading and recording systems are basically constructed of an image input apparatus, an image processing apparatus, and an image output apparatus. In the image input apparatus, image information of an original or subject which is fed to a given image reading position is scanned with a laser beam or the like, and the scanned image information is converted into an electric signal depending on the intensity of the reflected beam of the applied laser beam. Then, the photoelectrically converted image information is processed for gradation correction, profile emphasis, etc., according to platemaking conditions in the image processing apparatus. Thereafter, the processed image signal is converted back into a light signal such as a laser beam signal which is applied to and recorded on an image recording medium of a photosensitive material in the image output apparatus. The image recording medium with the image recorded thereon is developed by an image developing device and will be used as a film plate for printing.
If the image on an original is a continuous tone image such as a photographic image, a painting, or the like, then it is necessary to break up the image into closely spaced dots in order to express the shades of the image. More specifically, a continuous tone image is converted into a halftone dot image which comprises dots whose size depends on the density of the image. According to one method of breaking up a continuous tone image into halftone dots, a light image representing the continuous tone image is radiated onto a recording medium through a contact screen which has a vignetted dot pattern. In the image scanning and reproducing system, a dot screen signal corresponding to a contact screen is electrically produced, and a halftone dot image is generated on the basis of the dot screen signal.
The dot screen signal is composed of a threshold signal which has a number of gradations of tone corresponding to the resolution of the image scanning reading and recording system. A binarized halftone dot image signal is produced when the threshold signal and the image signal representing the continuous tone signal are compared with each other. If the number of gradations of tone of the threshold signal is small, the halftone dot image tends to suffer from a tone jump.
More specifically, FIG. 1 of the accompanying drawings shows the relationship between a threshold signal and the output density of an image which has been broken up into dots based on the threshold signal. Ideally, it is desirable for the output density of the image to have a linear range of gradations as indicated by the dot-and-dash line. Actually, however, the output density of the image which is broken up into dots based on the threshold signal by the image scanning reading and recording system is of discrete values as indicated at a.sub.1 through a.sub.6 depending on the diameter of a laser beam in the system. When the threshold signal ranges from 2 to 3 or 5 to 6, the output density of the dot-converted image is subjected to a large tone jump.
Such a tone jump may be suppressed if the number of gradations of the threshold signal is increased for a higher resolution. However, this solution is disadvantageous in that the required circuit arrangement is complex, the apparatus is expensive, and a long signal processing time is needed. Another proposal is to compare an image signal with a threshold signal to which randomly produced noise has been added, instead of increasing the gradations of the threshold signal, for producing a halftone dot image. Such a proposal is advantageous in that any tone jumps which are caused are relatively difficult to recognize but still fails to fully eliminate changes in the output density due to a low resolution.